In conventional 35mm film cassettes the filmstrip is wound on a flanged spool which is rotatably supported within a cylindrical shell. A leading or forward-most section of the filmstrip approximately 6 cm long, commonly referred to as a "film leader", protrudes from a light-trapped slit or mouth of the cassette shell. One end of the spool has a short axial extension which projects from the shell, enabling the spool to be turned by hand. If the spool is initially rotated in an unwinding direction, the film roll inside the shell will tend to expand radially since the inner end of the filmstrip is attached to the spool, and the leader section protruding from the slit will remain stationary. The film roll can expand radially until a firm non-slipping relation is established between its outermost convolution and the inner curved wall of the shell. Once this non-slipping relation exists, there is a binding effect between the film roll and the shell which prevents further rotation of the spool in the unwinding direction. Thus, rotation of the spool in the unwinding direction cannot serve to advance the filmstrip out of the shell, and it is necessary in the typical 35 mm camera to engage the protruding leader section to draw the filmstrip out of the shell.
A 35 mm film cassette has been proposed in U.S. Pat. No. 4,423,943 which, unlike conventional film cassettes, can be operated to thrust a film leader out of the cassette shell by rotating the film spool in the unwinding direction. The film leader originally is located entirely inside the cassette shell. A characteristic of this type of cassette is that the film strip becomes transversely bowed as it is uncoiled from the film spool. This facilitates movement of the film to a passageway leading to the outside of the cassette shell. A drawback of this kind of cassette is that severe transverse bowing of the filmstrip may damage the filmstrip.
Commonly assigned U.S. Pats. Nos. 4,834,306; 4,848,693; 4,883,235 and 4,948,063 each disclose a film cassette which requires no transverse bowing of the filmstrip, but which is capable of automatically advancing a non-protruding film leader to the outside of the film cassette shell in response to rotation of the film spool in the unwinding direction.
It has been found that, when the film leader is thrust through the light-tight passageway to the exterior of the cassette shell, the passageway presents some resistance to outward movement of the leader. This resistance may cause the leader to uncoil against the interior curved wall of the cassette shell. Consequently, increased torque will be required to rotate the film spool in the unwinding direction. This is a disadvantage.
The application of Niedospial, Ser. No. 506,320, filed Apr. 9, 1990, discloses a further improved film cassette wherein a pair of flexible flanges are coaxially arranged along the film spool to confine the ends of film roll coiled about the spool within respective skirted peripheries of the flanges. This prevents the film leader from uncoiling against an interior curved wall of the cassette shell. A film stripper proximate to a light-tight passageway to the exterior of the cassette shell is received between the film leader and the next-inward convolution of the film roll responsive to rotation of the spool core in the unwinding direction to remove the leader from the confinement of the skirted peripheries to permit it to enter the passageway. At least one of the flexible flanges is fixed to the spool core in the unwinding direction to effect a frictional relationship between the film leader and the skirted periphery of the flange, thereby improving the ability of the film spool to thrust the film leader through the light-tight passageway to the exterior of the cassette shell.
All film cassettes such as disclosed in the cited references, wherein a non-protruding film leader is pushed, propelled or thrust (instead of being pulled) to the exterior of the cassette shell are referred to herein as thrust-type film cassettes. These cassettes are in general, useful with conventional photographic roll films such as cellulose triacetate films of 35 mm size and conventional thickness. Conventional photographic roll film has a relatively high coefficient of friction between its emulsion side and back side. This has been advantageous in the manufacture of the film because the film is not so slippery as to cause large rolls of it to telescope or unwind during handling of the rolls.
U S. Pat. No. 4,866,469 relates to a preloaded single-use camera in which the unexposed film is pulled or drawn from a first holding chamber to a second holding chamber. Although the patentees discuss "coefficient of dynamic friction" between the front and back layers of a film as measured by a particular test, the patentees are not concerned with film cassettes. In particular, they have no suggestions with regard to film cassettes from which a photographic film is thrust rather than pulled from a supply chamber. The patent has no suggestion of the problems encountered in the functioning of a thrust cassette.